Magnetic nanocomposite for lead (II) removal from water

A magnetic perovskite-spinel oxide nanocomposite synthesized through a sol–gel self-combustion process is used for the first time as an adsorbent to remove toxic heavy metals (i.e., Pb2+). The synthesized LaFeO3:CoFe2O4 ((LFO)1:(CFO)x) (x = 0.11–0.87) nanocomposites possess good stability, abundant oxygenated active binding sites, and unique structural features, making them suitable for removing divalent Pb2+ ions. Scanning electron microscopy, X-ray diffraction, BET surface area, magnetization measurements, zeta-potential analyses, and X-ray photoelectron spectroscopy were used to analyze the nanocomposites, and their structural changes after Pb2+ ions adsorption. Batch tests confirmed that (LFO)1:(CFO)x efficiently removes Pb2+ from water with a maximum adsorption capacity of 105.96 mg/g. The detailed quantitative study indicates that the interaction of hydroxyl groups with Pb2+ ions occurs through electrostatic interactions and complex formation. We also demonstrate a new ring-magnetic separator system that allows magnetic separation of the toxic ions at a higher speed compared to traditional block magnets. The unique structure, high porosity, large specific surface area, and oxygenated functional groups of (LFO)1:(CFO)x nanocomposites make them promising materials for removal of heavy metal ions and possibly other environmental pollutants. This study provides a new approach to preparing nanocomposites of magnetic spinel ferrites with perovskite oxides for environmental applications.


Isotherm models for lead(II) adsorption on (LFO)1:(CFO)0.43
Langmuir isotherm equation 1 : 1 m a e e a e q K C q KC   (S1) Freundlich isotherm equation 2 : Redlich-Peteraon isotherm equation 3 : 1 e e g e AC q BC   where, qe is the sorption capacity of (LFO)1:(CFO)0.43 at equilibrium, Ce is the equilibrium concentration of Pb 2+ in solution, qm is the maximum adsorption capacity, and Ka, KF, n, A, B, and g are the isotherm constants for the Langmuir, Freundlich, and Redlich-Peterson isotherm model equations.
Kinetics models for Pb 2+ adsorption on (LFO)1:(CFO)0.43 where, qt is the metal ion concentrations at time t and k1 is the first-order rate constant (s -1 ).
where, k2 represents the second-order rate constant (g/mg/h); in this case, t/q is plotted against t.
The equilibrium rate constants of the linearized Lagergren pseudo first-and second-order kinetic models were expressed by plotting time t (h) against ln(qe-qt) and t/qt, respectively.

Computer-aided preliminary device design
Computational simulations were carried out using COMSOL Multiphysics software in order to design a prototype device for the magnetic separation of nanocomposites using an external magnetic field.
NdFeB ring permanent magnets with features based on commercial products (grade N48) were considered.In order to maximize the resulting overall magnetic force, which will be responsible for the magnetic separation of the material within the liquid, in the simulations a variable number of such NdFeB magnetic rings were arranged around a section of a glass tube.
Specifically, two setups were considered: 3 and 5 rings.For each of the two cases, a parametric study was conducted by varying the mutual distance of these rings in the range of 1/4 to 4 times the ring thickness (i.e., mutual distance from 2 to 32 mm).The magnetic polarization of the rings (always parallel to the glass tube axis) has been considered both concordant or alternate.
For each of the two cases, the overall induced magnetic force was evaluated while varying polarization and ring spacing.This value was calculated as the integral in the relevant channel section (blue part in Fig. S3) of the H-field square gradient radial component.This value represents the radial component of the net magnetic force, dependent on the nanocomposite characteristics, such as the particle volume   and its magnetic susceptibility   (equation S6) 4 .
Figure S3 shows a sketch of one of the two setups considered in the simulations.

S-6
Figure S3.Axisymmetric 2D representation of the computational domain for the 3-ring system.
Figure S4 shows the integral of the H-field square gradient radial component (i.e., the net radial component of the magnetic force induced by the magnets) for the 3-and 5-ring system as the distance between the rings varies.Only the case of concordant polarization between the rings is reported, being always better than the alternate case.Data are reported in terms of total value (Fig. S4a) and normalized value (Fig. S4b).The normalization is intended with respect to the contribution to the magnetic force generated by a ring, i.e., the ratio of the actual total value to the "theoretical" total value, understood as the product of the value of the single ring times the number of rings in the system.Interestingly, the obtained results demonstrate that the magnetic fields generated by the single rings interact either destructively or constructively as a function of the distance between them (Fig. S4b).Moreover, there is an optimal distance at which the synergistic effect given by the constructive interaction is maximized.This distance was found to be 24 and 20 mm for the 3-and 5-ring configuration, resulting in a gain of approximately 2% and 4%, respectively, compared to the simple additive contribution of the fields (Fig. S4b).For any given configuration, knowing the volume of the glass tube section affected by the rings, it is possible to derive the average value ∇  2 ̅̅̅̅̅̅ , which, in the optimal case of the 5-ring configuration is equal to 4.74  10 11 A 2 /m 3 .Given the size of a particle and its magnetic susceptibility, the average value of the magnetic force exerted on the particle (which changes at each point in the domain) remains defined by the above equation S6.

Figure S1 .
Figure S1.Field dependence of magnetization of LaFeO3 at T = 300 K.

Figure S4 .
Figure S4.(a) Volumetric integral of the radial component of ∇  2 determining the total magnetic force FM felt by the nanocomposites in the glass tube section affected by the rings; (b) percentage value of ∇  2 normalized with respect to the theoretical value generated by a single ring.

Table S1 :
La:Co ratios and magnetization at H = 18 kOe for the different nanocomposite samples.